Instead of having to implement certain interfaces in models, it would be good to support parallel classes that provide the required meta data. So for model pakkage.Foo, pakkage.FooMetaData will contain the constraints and validation declarations. Like this the models are totally RIFE agnostic. The difficulty for the implementation is that constraints are tied to model instances and not model classes, so RIFE has to ensure that the meta-data instances are always used by the same model instances.